The timing of the biological processes of organism is controlled by an endogenous circadian clock. The molecular clock is present in almost every cell and is synchronized with the external environment. The main mechanism of the clock is a transcription-translation feedback loop. The 24-hour circadian rhythm period is provided by reversible posttranslational modifications (PTMs) of the clock proteins and another regulators of the circadian clock. PTMs are further important for clock entrainment, their regulation by metabolic state in the cell, and reciprocal regulation of the circadian clock end cell cycle. Phosphorylation, histones PTMs, acetylation, SUMOylation, ubiquitination, O-linked N-acetylglucosamination and polyADP-ribosylation play a crucial role. The molecular mechanism of the biological clock is an evolutionarily conserved mechanism found in most organisms. This bachelor thesis summarizes the knowledge about the role of PTMs in the molecular mechanism of the mammalian and human circadian clocks. Key words: circadian clock, clock genes, clock proteins, posttranslational modifications
